Electroacoustic transducer



Aug. 29, 1950 Filed Sept. 9, 1948 FIG.

PRESSURE RESPONSE IN DEC/EELS zobo moo INVENTOR RR. KRE/SEL ATTORNEYPatented Aug. 29, 1950 ELECTROACOUSTIC TRANSDUCER Ralph R. Kreisel, NewProvidence, N..J., assignor -to Bell Telephone Laboratories,Incorporated,

New York, N. Y., a corporationof New York Application september 9, 1948,Serial N 0. 48,382

13 Claims.

This invention relates to electroacoustic transducers and moreparticularly to such devices of the ring armature type.

In ring armature type transducers, such as disclosed in Patent2,249,l60, granted July 15, 1941 to Edward 'E. Mott and the applicationSerial No. 755,397, filed June 18, 1947, now Patent No. 2,506,624,granted May 9, 1950, of Robert E. Wirsching, the vibratory elementcomprises a non-magnetic diaphragm joined at its periphery to theinner-marginal part of an annular or ring armature which is supportedadjacent its outer margin or periphery. Advantageously, in order thatthe restraint upon Vibration of the element may be substantially aminimum, the armature is unclamped. In general, it is seated upon asupport, for example, upon apart of the magnetic structure, and has itsperiphery adjacent and slightly spaced from a surrounding element whichserves to prevent undue lateral displacement of the armature. v

It has been found that ,in use of the transducer, for example asincluded in a telephone handset, lateral displacement of thearmaturediaphragm unit may occur as a result of jars or mechanicalshocks to which the transducer is subjected. Asa consequence of suchdisplacement, the armature contacts and may remain in edge contact withthe surrounding element. This introduces frictional forces which producea degradation of the performance of the device,

specifically a decrease in efiiciency and distortion of the freguencyresponse characteristic,

One object of this inventionis to prevent such degradation inperformance.

'More specifically, an object of this invention is tomaintain thediaphragm-armature element at or substantially at its optimum positionwithout imposing substantial additional restraint upon vibration of thiselement.

In accordance with one feature of this invention, ina ring armaturetransducer of the general construction described hereinabove, means areprovided for minimizing or'substantially preventing-lateral displacementofthe armaturathc means-being constructed and arranged to maintain-thedesired optimumposition of the armatore-diaphragm element withoutsubstantially affecting the vibratory characteristics of this element.

In one illustrative embodiment of this invention, the-armature-diaphragmelement is provided'with thin, lightweightprojections extendingoutwardly beyondthe periphery of the armature into proximity to or inlight contact with the element surrounding the armature. Theseprojections, in one advantageous construction, may be narrow radiallyextending fingers integral with the diaphragm and widely spaced so thatthe mechanical restraint to armature-dia phragm vibration introduced bycontact of the fingers with the surrounding element is negligible froma. practical standpoint. These fingers may be aflixed to the armature orfree to flex independent thereof.

The invention and the above-noted and other features thereof will beunderstood more clearly and fully fromthe following detailed descriptionwith reference to the accompanying drawing in which:

Fig. 1 is a side sectional view of a telephone receiver illustrative ofone embodiment of this invention;

Fig. 2 is ,a partial plan View of the armaturediaphragm .elementincluded in the receiver shown in Fi 1;

Fig.3 is an enlarged fragmentary view in section showing details of thearmature-diaphragm element;

.Fig. 4 isa view similar toFig. 3 illustrating another embodiment ofthis invention; and

Fig. 5 is a graph illustrating certain substantive features oftransducers constructed in accordance with this invention.

Referring now to the drawing, the nature of thedegradation inperformance of ring armature transducers Which may result from lateraldisplacement of the armature therein is. illustrated in Fig.5. Inthisfigure, curve A represents the response of a ring armature receiverwherein the periphery of the armature was spaced from the surroundingelement and curve B represents the response of the same receiver whenthe diaphragm-armature unit therein had become displaced lateraily sothat a portion of theperiphery of the armature was in contact with thesurrounding element. It is evident that as a result of the contactbetween the armature and the surrounding ,member'not only is theamplitude of the response decreased, several decibels in typical cases,over a wide rangeof frequencies, but also substantial non-uniformity ordistortion is introduced at the higher frequencies, for example,particularly in the range between about 1500 and 2800 cycles per second.

In accordance with'this invention, such degradation of performance issubstantially prevented. The telephonereceiver,illustrative of oneembodiment of this invention, shown in Fig. 1 includes a magnetic systemcomprising a pole-piece ample of metal or plastic, comprises acylindrical portion M fitted within the magnet portion [2 and anintegral annular flange I5 seatedupon and affixed to the base flange 1Bof the polepiece. Seated upon the support is and fitted.

freely within the cylindrical portion [2 of the magnet is an annulararmature I6, of a material having a high magnetic permeability, whichhas its inner marginal portion coaxial with and in juxtaposition to theface of the pole tip portion i I- The armature is held in position uponthe support by magnetic'forces.

The armature l6 mounts a'lightweight nonmagnetic diaphragm, for exampleof a fabric impregnated with a phenolic condensation product,'having adished central portion ii and an integral flange portion l8 which isformed with spaced protuberances do to prevent sticking of the armatureto the magnet flange l3.

A centrally dished and apertured grid-ferrule ,isclamps a dust andmoisture excluding membrane 23' against the flange l3, and also servesas a clamping band to hold firmly together the several parts of thestructure in pile-up relation, specifically the magnet, pole-piece, andthe insulating terminal plate M. The ends of a cylindrical signal coil23 are connected to terminals 58 which are mounted on the terminalplate.

In the construction of thesub-assembly of the pole-piece I9, I l and thesupport M, [5 the tip face of'the pole-piece portion H is machinedunderflush with the plane defined by the free end of the support !4, sothat a prescribed normal air gap is provided between the armature iiiand the pole tip face when the armature is seated upon the support M. IAs shown clearly in Fig. 2, the flange l8 is pro- 'vided with aplurality of equal spaced projections or tabs .24 which extend radiallyoutward beyond the periphery of the armature it. These projections ortabs may terminate slightly short of the inner wall of the cylindricalmagnet portion [2 or may make light contact with this portion. In eithercase, it will be appreciated that the tabs serve to position thearmature laterally and to prevent contact between the armature and themagnet portion 12. The tab-s are of thin flexible material and it isevident that the area of the contacts between the outer ends thereof andthe magnet is small. Hence, the forces effective upon thediaphragm-armature vibration due to contact between tabs and magnet isvery small and negligible for practical purposes. during use of thereceiver and even if it is subjected to shocks and jars, thearmature ismaintained 'in proper position relative to the magnet and pole-piece,and this is realized without the imposition of substantial restraintupon vibration of the armature-diaphragm assembly or unit. Consequently,degradation of the performance of the receiver by mechanical shocks andjars is prevented.

Ifhe projections or tabs 24may be free of'the armature or may be afiixedthereto throughout 4 r the major portion of their lengths. In the formercase, as illustrated in Fig. 3, the flange l8 of the diaphragm isaflixed to the armature by a thin layer Or washer 25 of cementingmaterial. In the latter case, illustratedin Fig. 4, the flange l8 andthe tabs 2@, except for the extreme outer portions thereof, are affixedto the armature by a layer or washer of cement 26A.

Although specific embodiments of the invention have been shown anddescribed, it will be understood that they are but illustrative and thatvarious modifications may be made therein without departing from thescope and spirit of this invention as defined in the appended claims.

What is claimed is: 1. An electroacoustic transducer comprising amagnetic system including an annular armature,

' a support mounting said armature adjacent one of its margins, aretaining member adjacent and spaced from the other margin of saidarmature,

said armature being held vupon said support by magnetic forces, andmeans for preventing contact between said armature and said retainingmembercomprising spaced members coupled to said armature and projectingbeyond said other margin toward said retaining member.

2. An electroacoustic transducer comprising a magnetic system includingan annular magnetic armature, a support'adjacent the periphery of saidarmature, said armature being seated upon said support and held thereonby magnetic forces, a fixed member disposed about said periphery andspaced therefrom, and means for preventing contact between said armatureand said'fixed member comprising laterally spaced flexible membersconnected to said armature and projecting outwardly beyond saidperiphery.

3. An electroacoustic transducer in accordance with claim 2 wherein saidflexible members overlie and are spaced from one face of said armature.4. An electroacoustic transducer comprising a support having a seatingsurface, a fixed member adjacent and disposed about the periphery ofsaid surface, a diaphragm armature unit including an armature seated onsaid surface and having its periphery adjacent and spaced from saidfixed member, and laterally spaced projections on said unit and engagingsaid fixed member.

5. An electroacoustic transducer in accordance With claim 4 wherein saidunit includes a nonmagnetic diaphragm and said projections extend .fromsaid diaphragm over one face of said armature.

6. An electroacoustic transducer comprising a support having a seatingsurface, a magnetic system including an annular armature seated uponsaid surface, a retaining member disposed about the periphery ofsaid'armature and spaced therefrom, a diaphragm mounted by saidarmature, and means for preventing contact between said armature andsaid retaining member comprising fingers projecting from said diaphragmadjacent one face of said armature and beyond the periphery of saidarmature.

7. An electroacoustic transducer comprisinga support having a seatingsurface, a retaining member encompassing said support, and a vibratoryelement comprising an annular armature and a non-magnetic diaphragmaflixed to the inner marginal part of said armature, said armature beingseated, adjacent its outer margin, upon said surface and having itsouter margin spaced'from said retainin 'member, said diaphragm havingportions projecting outwardly beyond said outer margin of said armature,

8. An electroacoustic transducer comprising a support having a seatingsurface, a retaining member encompassing said support, a vibratoryelement comprising an annular armature and a non-magnetic diaphragmaffixed to the inner marginal part of said armature, said armature beingseated, adjacent its outer margin, upon said surface and having itsouter margin spaced from said retaining member, and means for preventingcontact between said armature and said retaining member comprisingextensions of said diaphragm spaced circumferentially over and adjacentone face of said armature and projecting beyond said outer marginthereof.

9. An electroacoustic transducer in accordance with claim 8 wherein saidextensions are narrow flexible fingers which are spaced from said oneface of said armature.

10. An electroacoustic transducer in accordance with claim 8 whereinsaid extensions are narrow, flexible fingers secured to said armature.

11. An electroacoustic transducer comprising a magnetic system includinga cylindrical polepiece, a magnet having a cylindrical portion spacedfrom, encompassing and coaxial with said pole-piece and an annularmagnetic armature having its inner margin adjacent said pole-piece andits periphery in juxtaposition to but spaced from said cylindricalmagnet portion, means mounting said armature adjacent its periphery,said armature being held upon said mounting means by magnetic forces,and a non-magnetic diaphragm having a central portion the marginal partof which is afiixed to said armature adjacent the inner margin thereof,said diaphragm having integral flexible fingers extending adjacent oneface of said armature and beyond the periphery of said armature.

12. An electroacoustic transducer comprising a support, a vibratilemember including an annular armature, a magnetic system including saidarmature, said armature being seated upon said support, held thereon bymagnetic forces and laterally displaceable upon said support, a retainermember encompassing said armature and normally spaced a preassigneddistance from the periphery thereof, and means including cooperatingportions of said vibratile and retainer members for limitingdisplacement of said armature toward said retainer member to an extentless than said preassigned distance.

13. An electroacoustic transducer in accordance with claim 12 whereinsaid portions of one of said members are fingers on said one member,extending toward the other of said members.

RALPH R. KREISEL.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,220,669 Miller Mar. 27, 19172,249,160 Mott July 15, 1941 FOREIGN PATENTS Number Country Date 250,719Great Britain Apr. 22, 1926

